This invention relates to an acoustic vibrational material formed of a fiber reinforced epoxy resin and used for a speaker, a diaphragm or the like.
Various properties are required of the acoustic vibrational materials used in speakers, diaphragms, etc. in pursuit of a reproduced sound of a higher quality. One of these is the larger longitudinal wave propagation velocity [=(specific modulus of elasticity).sup.1/2 =(Young's modulus/density).sup.1/2 ]. To this end, a material having a higher Young's modulus and a lesser density is preferred. Simultaneously a larger internal loss is also required for absorbing the abnormal sound or distortion component produced with divisional vibrations.
It is however customary that the material having a higher Young's modulus and a larger longitudinal wave propagation velocity undergoes a lesser internal loss so that it is difficult to satisfy these contradictory conditions simultaneously in a single material. Therefore, the present situation is that one of these properties has to be preferred depending on the usage, such as low sound reproduction or high sound reproduction.
It has recently been attempted to solve the above problem by the combination of several materials. An example is the use of a composite material having various fibers, such as the carbon, aramid, glass or polyolefin resin fibers, as the reinforcement material. Above all, attention is directed to ultra-drawn polyethylene fibers as the fiber reinforcement material because they undergo larger internal losses and are superior in transient characteristics while being lesser in the characterization of the playback sounds. In most cases, it is used in a composite form with epoxy resin.
However, the above mentioned composite materials cannot be said to ensure the practical properties satisfactorily. Although the fiber reinforcement material consisting of a material undergoing larger internal losses has recently been evolved, the present situation is that, since the epoxy resin compounded with the material is not sufficiently high in internal losses, the shortage in the internal losses has to be compensated by constructional artifices in the diaphragm or fine adjustment of various parts after speaker assembling.